In a plasma cutting or welding process, especially where a workpiece or a pair of workpieces having a configuration in which the accessibility of a plasma torch thereto is poor, it has been necessary to employ a plasma torch having an elongated forward end.
If, however, such a plasma torch having an elongated forward end is employed, a problem has been encountered a large welding or cutting current a torch nozzle tends to be melted and damaged due to an insufficient cooling thereof. The required cooling effect does then become insufficient since it is unable to ensure a sufficient water coolant passage up to a point of the arc restraining region or the forward end of the said torch nozzle that tends to be elevated in temperature by an arc heat as well as by a radiation heat from the work due to the dimensional limitation of the said plasma torch.
In an attempt to meet with this problem, there has hitherto been proposed a modified plasma torch, as disclosed in Japanese Examined Patent Publication No. Sho 62-47630 or Japanese Examined Patent Publication No. Sho 63-39347, that is designed to make its forward end flat, thereby enhancing its accessibility to a workpiece in two side directions, and at the same time to provide a cooling water passage around a torch nozzle, thereby enhancing the ability to cool the torch nozzle.
Such a plasma torch in the prior art has its forward end flat in horizontal cross sectional, approximately rectangular, configuration and its torch nozzle whose axis is located as coincident with a center line of its torch body. And, the said plasma torch has a cooling water feed passage and a cooling water return passage distributed at both sides of the said torch nozzle, the two passages being configured to communicate with each other at the forward end of the said torch nozzle so that said both sides and said forward end of the torch nozzle may be cooled.
There has also been proposed in the prior art a plasma torch of another configuration, as disclosed in Japanese Unexamined Utility Model Publication No. Hei 1-60783, in which an electrode thereof is fitted and supported in an electrically conducting section of its torch body and a torch nozzle is disposed outside of this electrode via a spacer (constituted of a guide cylinder) of an insulating material. And, in it, the shortest gap section between the electrode and the torch nozzle is provided in the vicinity of an electrode forward end so that a dielectric breakdown due to a high frequency voltage which develops when a pilot arc is ignited may occur in the said shortest gap section (in the vicinity of the electrode forward end) while preventing an abnormal discharge which might otherwise be produced in any area other than the electrode forward end within the torch body.
Also, there has been a plasma torch of still another design in the prior art, as disclosed in Japanese Unexamined Patent Publication No. Hei 5-379, Japanese Examined Patent Publication No. Sho 56-4351, which is provided with a cap that is attached to an electrical insulator mounted to surround the forward end of its torch body whereby a welding or a cutting operation is carried out while maintaining a standoff constant by holding the said cap in contact with a workpiece.
It may be noted that a conventional plasma torch of "flat" type as mentioned above has the advantage that the closeness of the outer surfaces of both sides in two directions of the thinner portion of its horizontal cross sectional configuration from the axis of the torch nozzle leads to a good accessibility to a workpiece at these both sides, which in turn leads to the ability to weld while displacing towards the thicker portion of the said cross sectional configuration. The plasma torch can even be inserted even into a groove section if the root faces of a groove edge butt section of thick plates are to be welded together. With this construction, however, it should be noted that since the accessibility to the workpiece is only good at the both sides of the said thinner portion of the cross sectional configuration, applicable workpieces are limited; furthermore, if the plasma torch furthermore if mounted on a robot, and a rectilinear welding operation is to be performed there will be a number of singular points generated which are impractical in a posture of the robot. Hence, there arises a limited flexibility. Furthermore, there arise the problems that such a plasma torch in the prior art, with the torch nozzle as a consumable part being of a flat configuration, may make an entire machining process for a given purpose complicated, may require a brazing step, thus making the process highly expensive, and will necessarily make a running cost hereof prohibitively high.
Also, in a plasma torch in the prior art of the type mentioned above the shortest gap section between the electrode and the torch nozzle is located in the vicinity of the electrode forward end, and is designed to dispose an outer torch nozzle around the electrode forward end via a guide that is composed of an insulating material. Further, such a plasma torch is designed to dispose, in the vicinity of the electrode forward end, the shortest gap section between the electrode and the torch nozzle so that a dielectric breakdown with a high frequency voltage that develops when a pilot arc is ignited may occur in the said shortest gap section (in the vicinity of the electrode forward end), thereby preventing a discharge (abnormal discharge) that might otherwise occur in an area other than the electrode forward end. However, since there should naturally be a space (gap) produced between the said guide (of an insulating material) and the torch body, especially if the said guide is leaked with a water coolant or in the like situation, the problems arise that when the pilot arc is generated, a so-called creeping discharge will be generated over the guide surface. Thus, a discharge (abnormal discharge) will develop between an area other than the electrode forward end and the torch nozzle to the extent that the torch nozzle may be burnt or otherwise damaged.
Alternatively, another plasma torch in the prior art in which the torch body has a cap attached to its forward end, has said cap attached to the electrical insulator mounted around the forward end of the torch body and is designed to carry out a welding or a cutting operation while maintaining a standoff constant by holding the said cap in contact with a workpiece. This enables an arc length to be held constant and the cutting or welding quality to be stabilized. Also, if this is used in welding, it has been noted that especially in an arc spot welding process, since a welding spot can be covered with the cap attached to the forward end, there ensues an enhanced shielding effect so that such a welding spot may remain unoxidized. With any of these plasma torches, however, since the forward end cap is directly in contact with a workpiece, certain problems are not avoidable. Thus, due to a heat conduction from the workpiece or by receiving a radiation heat from the arc, the cap tends to be melted and deformed. This results in the inability to maintain the standoff constant and causes the torch itself to be melted and damaged. Also, in a plasma torch in which a plasma gas is swirled, the problem has been found to develop that depending on a particular shape of a gas venting hole provided, if a workpiece arc spot is shielded, an air may be entrapped into a chamber constituted by the torch body and the cap to oxidize arc spot or a flow of the gas in the chamber may be disturbed, thereby lowering welding quality.
The prevent invention has been made with the foregoing problems in consideration.
It is a first object of the present invention to provide a plasma torch whereby where an electrode and a torch nozzle, especially the forward end of the torch nozzle and an arc restraining section or its vicinity that receives a radiation heat from an arc or a working zone, are enough cooled by a water coolant, the lengths from the axis of the torch nozzle in at most three directions are minimized, the accessibility to a workpiece in the said directions is predominantly enhanced while providing less costly. The electrode and the torch nozzle which are regarded as consumable parts and are readily machinable since they are arranged to be symmetrical to each other.
Also, it is a second object of the present invention to provide a plasma torch whereby a creeping discharge path of a guide surface within a torch body is blocked, thereby preventing the occurrence of an abnormal discharge within the torch and preventing the torch from being burnt and otherwise damaged.
It is a third object of the present invention to provide a plasma torch which is capable of preventing a workpiece engaging or contacting cap attached to the forward end thereof from being melted and deformed by permitting it to be cooled by a water coolant in order to maintain a standoff constant, permits a workpiece arc illumination peripheral region to contact with the said cooled workpiece contacting cap and to be cooled thereby and, especially where a lap spot welding operation is to be performed, permits the diameter of a molten pool on the workpiece front side to be minimized so as to enhance the appearance quality of the welded workpiece, and further permits an arc illumination area to be shielded completely from the outer atmosphere while preventing a flow of the plasma gas from being disturbed so as to enhance the welding quality and stability.